A steel plate is continuously thermally treated in a heating furnace to remove residual stress and passes through melted zinc in a plating pot to be plated while remaining at an adequate temperature. The steel plate passes through a sink roll and a stabilizing roll provided in the plating pot and then passes by air knives disposed above the plating pot. A plating amount of the steel plate is adjusted to be a desired plating amount by a consumer through the air knives.
When the steel plate passes through the air knives, not only zinc scattering but also top dross that is zinc oxide on a break surface of the plating pot are formed due to a high-pressure gas injected from the air knives and oxidation of a melted zinc plating layer attached to a surface of the steel plate. When the top dross is attached to a surface of a transferred steel plate, surface defects such as stabbed dross are caused. Accordingly, it is very important to efficiently remove top dross.
Particularly, in operating at a high speed of 160 mpm or more, since an amount of generated top dross rapidly increases and then a worker should focus on an operation of manually removing the top dross 60% to 70% or more of overall operation, there is a problem in which workability is notably reduced.